Photoconductive organic materials for the near-IR radiation range

Polymer composition films based on polystyrene and containing electron donors, viz., substituted tetrathiafulvalenes, an electron acceptor, viz., 2,4,5,7-tetranitrofluoren-9-one, and sensitizers, viz., a cationic polymethine dye, 1,3,3-trimethyl-2-[3-(1,3-dihydro-1,3,3-trimethyl-2H-indol-2-ylidene)prop- 1-en-1-yl]-3H-indolium tetrafluoroborate, and a neutral mero- cyanine dye, 5-{3-[(1,3-dihydro-1,3,3-trimethyl-2H-indol-2-ylidene)ethylidene]-2-phenyl- cyclopent-1-en-1-ylmethylene}-2-thioxodihydropyrimidine-4,6(1H,5H)-dione, were prepared. The internal photo effect upon film irradiation in the near-IR range is provided by low ionization potentials of the donors and the high electron affinity of the acceptor. The photoconductivity increases upon replacement of a cationic dye by a neutral one, mainly because of photo generation of mobile charge carriers of both signs and a decrease in the activation energy for the photoconduction current. The decrease in the activation energy for the photocurrent is due to the fact that in the case of a neutral dye, the mobile charge carriers move away from each other during their separation, while in the case of a cationic dye, the colorless counter ion strongly holds the photo generated charge carrier.